A Perl program using fork() emulation on Windows may end up
kill()ing the forked child and exiting immediately. There
is a race condition where the process exit code may be
changed to '9' (the signal used to kill the child thread),
overriding the value the parent thread used in ExitProcess()
(called implicitly with the return value of main()).

Giving up the remainder of the time-slice after terminating
a child thread seems to eliminate this race.

This bug is responsible for various CPAN test failures,
where all tests seem to pass, but Test::Harness still
reports: "Dubious, test returned 9 (wstat 2304, 0x900)"
(e.g. HTTP-Server-Simple, tests based on Test-TCP).